1. Field of the Invention
This invention relates to a hydrulic booster and, in particular to a booster for an anti-lock vehicular brake system of the type having a booster piston with one end surface confining a booster chamber and another end surface confining a resetting chamber and also, having a pedal-force-actuated braking-pressure control valve for delivering pressure into the booster chamber which is proportional to the pedal force.
2. Description of the Relevant Art
A booster of this type is described in the German publication DE-OS 34 44 828. The booster is part of a hydraulic anti-lock brake system. The wheel brake cylinders are in communication with the master brake cylinder by way of closable brake lines. For the purpose of developing pressure in the wheel brakes, the master brake cylinder is assisted by the hydraulic booster, and the master brake cylinder pressure is supplied through the open brake lines to the wheel brake cylinders. Pressure modulation in a wheel about to lock during braking is achieved as follows: first the brake lines to the non-locking wheels are shut off, and subsequently the master brake cylinder is relieved from the actuating force. As a result, the pressure in the master brake cylinder and also in the wheel brake cylinder of the imminently locking wheel is decreased. To relieve force of the master brake cylinder, a counterforce is built up in the booster provided upstream thereof. The hydraulic booster includes a booster piston which confines the booster chamber with one end surface and bounds the resetting chamber with its other end surface. For actuation of the master brake cylinder, a pressure proportional to pedal force is built up in the booster chamber which acts upon the booster piston and displaces it in a direction for actuation of the master brake cylinder. In order to reset the booster piston (i.e., for relieving the master brake cylinder from force) the following connecting operation is performed. The resetting chamber, which is initially pressureless, is connected to the pressure side and the booster chamber is connected to the suction side of the pump. The resetting chamber fills with pressure fluid and the booster chamber is evacuated. For renewed pressure build-up, the switching valves are reset so that the booster chamber is again filled with pressure fluid and the resetting chamber is evacuated.
Since braking pressure modulation comprises several control cycles and the booster piston is moved back and forth considerable quantities of pressure fluid are required which must be made available by the pressure source. This is possible only in the event that the pressure source--by means of a pump--is always replenished with the pressure fluid that is discharged in each case. Therefore, the pump must have a large feed capacity and, accordingly, has a correspondingly great energy requirement.